In both historic and modern times, bookbinders have commonly used leather as a covering material. While full, half and quarter-bound leather volumes are commonplace in collections and on book conservator’s benches, leather also plays a role in other functions of book construction where strength and flexibility are desired. Leather is an organic material which is formed by chemically altering animal hides at the molecular level to render them imputrescible, meaning that they are resistant to decay which would normally beset an aging animal hide. While other, physical, alterations such as scrapping, oiling, and stretching can be part of the creation of a processed animal hide, these steps alone cannot “tan” leather, and thus create materials like vellum and parchment. While these materials have some similarities to leathers, there are also many differences. This chapter pertains to the materials that are used for form leather, the chemical composition of untanned hides and the chemical alteration of both the tanning and tawing processes, the degradation of tanned hides, and common conservation treatments for leather. For a thorough discussion of similar topics pertaining to untanned hides such as vellum and parchment, please refer to the Paper Conservation Catalog chapter Parchment (PCC).

Sources of Leather

Leather is created from animal hides. While almost any hide can be tanned, a select few species of animal are commonly used for bookbinding. Calf, pig, sheep, and goat hides are the most traditional sources of bookbinding leather. These have been selected through the years for this purpose due to the workability of the final leather product for the needs of bookbinders, but also due to their ideal size to cover a complete quarto.

Other hides, such as larger and smaller mammals, reptiles, fish, and even humans, have been experimented at one time or another for bookbinding materials, though none are found in collections in great quantities, nor do they match the traditional sources in durability and utility for their purpose.

Calf

Goat

Pig

Sheep

Chemical Composition of Untanned Hides

Leather is composed of collagen-based animal hides that have been chemically altered by tanning, leathering, tawing or other means to render them imputrescible, have desired handling and working characteristics, and increase chemical and physical durability over time. To investigate the chemical composition of leather, it is best to first understand the chemical composition of skins and hides and how tanning alters these structures.

Collagen Molecules

Collagen is a protein molecule built of sequential chains of amino acids twisted and bound to form a strong, fibrous molecular structure. The amino acid monomers that are the basis of the collagen protein are composed of a carboxyl and an amino group and a variable side chain off a central carbon (see Fig. 1).

Fig. 1: Basic structure of an amino acid

These side chains, which give each individual amino acid its unique chemical characteristics, can range from a simple hydrogen to reasonably large functional groups that can be polar or non-polar, acidic or basic, aromatic or aliphatic. Non-polar side chains involve only carbon and hydrogen atoms, however polar side chains frequently involve carbonyl and hydroxyl groups, amino and amide groups, or thiols (also called mercaptans, -SH). The different amino acids are linked together by a covalent peptide bond formed by a condensation reaction between the carbonyl group of one amino acid and the amino group of another (see Fig. 2) to form a polymer chain called a polypeptide.

Fig. 2: Condensation reaction of two amino acids to form a peptide bond

Polypeptide Chains, Procollagen and Tropocollagen Structure

Collagen’s backbone, the polypeptide strand, is formed by a known twenty different amino acids that form a chain of about 100 units in length. In the chain, a common sequence of amino acids is glycene-X-proline or glycene-X-hydoxyproline, where X is a range of other commonly occurring amino acid residues. Hydroxyproline, an amino acid found in all collagen molecules (see Fig. 3), is rare in almost all other protein structures and its presence used as an indicator for collagen.

Fig. 3: Hydroxyproline

The procollagen structure is formed by the twisting together of three left-handed helical polypeptides into a triple helix with a right handed twist with three amino acid groups per twist. From this, the terminal extension peptide groups (found at each end of the polypeptide chain) are removed by specific proteases to form non-helical telopeptide regions thus finalizing the formation of the tropocollagen structure. This final quaternary structure is stabilized by multiple hydrogen bonds between the amino and carboxyl groups of adjacent helices. Due to the necessity of a tight helical structure, all large functional groups on amino acids are oriented to the outside of the helix.

Fibril and Fiber Structures

Collagen is a multiheirarchical structure which is further developed from the collagen molecules, resulting in four levels of macromolecular structure: first the molecules pack together into an organized secondary helical structure called a fibril, then those fibrils further organize into larger bundles called fibril bundles, then into fascicles, and finally into fiber bundles. Fibrils are the first level of the collagen structure that is visible via scanning electron microscopy (SEM).

The collagen fibril is stabilized by the formation of two types of chemical bonds: Salt links and covalent intermolecular bonds. Salt links are formed between acidic and basic functional groups on the amino acid side chains (see Fig. 4) whose strength is maximized by aligning polar regions of the fibrils. Covalent intermolecular bonds are formed by staggering the telopeptide regions (the terminal non-helical areas of the tropocollagen structure described above) with helical portions of adjacent molecules, thus resulting in a long fiber structure with no weak points.

Chemical Alterations Through Tanning

In order to achieve many of the physical changes to make a hide into leather,many chemical changes must also take place. The following is an overview of the chemistry of the major tanning processes, vegetable tanning, mineral tanning, brain tanning, aldehyde tanning and syntans. For the purposes of this chapter, vegetable tanning is the most common leather used, both historically and in modern times, for book binding. However, other tanning processes and non-tan processes, such as tawing will also be covered. For a summary of the production of vellum and parchment, refer to the Paper Conservation Catalog chapter Parchment (PCC).

Vegetable Tanning Chemistry

Classes of Tannins

Hydolysable/pyrogallol, which can be subclassed to

Gallotannins (tannic acid/sumac)

Ellagitannins (chestnut/oak)

Are sugar based – mostly glucose, though some larger polysaccharides [see Figure 5]

Raise the Ts to 75-80 C

Presence of a trihydroxyphenyl moiety (is this part of the tannin, I assume?) allows for the complexation of metal ions, resulting in a semi metal tan

Hydrolysable tannins break down by hydrolysis, then depositing esterifying acids with in the fiber structure – called a “bloom” (is this good or bad?)

Very reactive tannins due to the high number of hydroxy groups (on the collagen or on the tannin?)

Additionally the polyphenols can fix to the amino and carboxylic acid grps on the AmAc side chains.

Condensed tannins also can form covalent bonds between the collagen molecule and the aromatic carbon groups in the tannins via “quinoid” structures (see examples, below) which are more stable than H+ bonding and accounts for the increased Ts for condensed tannins. [see Figure 7]

[insert Figure 7: Quinoid Structure]

Mineral and Other Tans

Historic

Mineral Tanning Chemistry

Most effective is chromium II, which can raise Ts to over 100C

Other minerals that also tan, but not as well, are: aluminum, silicates, polyphosphates, titanium III and IV, zirconium, iron III

Chrome tanning

Chromium III interacts with collagen through the ionized carboxy grps (those with aspartic and glutamic side chains)

To maximize tan, intitiated at pH 2.5-3 w/ 33% basic chromium III sulphate. This allows for better penetration by the chromium species

pH is then gradually raised to 3.5-4, which increases the number of free rxn sites on the collagen while it increases the size of the chromium species (not sure what’s involved in the species).

Temp also elevated during tanning to produce maximize chromium content.

Masking of chromium III by use of ligands (molecule which generally donates one or more of its electrons through a coordinate covalent bond to, or shares its electrons through a covalent bond with, one or more central atoms or ions) of monodentate or bidentate ligand salts, which allow for a reduction of reactivity by bonding with reactive sites

Aluminum III taws

Titanium

Zirconium

Oil and Brain Tanning Chemistry

Oil tans - chamois

Brain tans – buckskin

Modern

Aldehyde tanning

Syntans – often used in combination with other tanning processes

Auxiliary

Combination/retanning

Replacement

Leather Manufacture

Leather can be produced through tanning or tawing (though tawed skins are not correctly called “leathers”, we will consider them as such in this chapter since they are used in much the same way as tanned hides). Tanning is accomplished through a series of sequential steps which are altred slightly depending on whether you are considering a historic or modern tanning process and also what type of tan is used. The most common type of tannage used in bookbinding leather is vegetable tanning. The most common type of tannage used in the modern production of leather is chrome tanning - a form of mineral tan - may be found on limited bookbindings, but is generally not found workable by bookbinders. Whereas alum tawing – a mineral tan using aluminum salts – was used frequently on historic book bindings, particularly those in the 16th and 17th centuries. Other processes such as oil and brain tanning to produce chamois and buckskin, or modern chemical tans such as aldehyde tanning or the use of syntans are also part of both historic and modern leather manufacture, but are again not used frequently in bookbinding, and will therefore only be touched on briefly. For each major process, the steps followed in the pretanning, tanning, and finishing stages are each described.

Vegetable Tanning

Historic Process

The process of historic vegetable tanning is perhaps the best documented and related the most closely to tannages used most commonly on historic bindings. During pretanning, all or some of the following steps were performed on an often partially-putrified hide to render it ready for tanning:

Pretanning

Putrefication brought about the decay of non-collagen materials in the raw pelt. This was accelerated by the addition of “biologically active” materials such as beer, urine, dung, or other fermenting matter, or alkaline (lime) baths, or a combination of lime baths with biologically active matter.

Hair removal was eased by thorough putrefication. To remove the hair a dull double handled knife was used to scrape the hair side of the skin once the follicles were loosened.

Fleshing was performed on the flesh side of the skin, which was again enhanced by putrefication. This was done by using a sharp, double-handled knife to scrape away small bits of clinging flesh from the pelt.

Scudding was the careful scraping of grain surface (hair side) of the pelt to remove debris left on the skin once the hair was removed.

Trimming was done on the sides of the pelt, by removing any small unwanted tags or bits of skin.

Rounding was a term used for the sorting of the hides, and trimming down if necessary, to parse them into different piles for various qualities of leathers and different tanning treatments.

Deliming was done if the skin had originally been treated with lime (or “limed”) as part of the putrefication.

Bating was the practice of softening the pelt by immersing it in baths of dog or pigeon dung, which partially digests the remaining protein structure in the hide and thus “opens up the flesh” for softer drape.

Drenching (also called Raising) is similar to bating, which involved the fermentation of the pelt in soured grain matter with Lactobacillus sp. bacteria. Though the use of their enzymes, these bacteria digest the souring grain to produce lactic and acetic acid and off-gas carbon dioxide. This forms bubbles of CO2 within the fiber network of the pelt while other enzymes digest the mucopolysaccharides in the ground, helping to break it down and increase the absorption of tanning agents into the fiber network.

Scraping again follows if any bating and drenching steps were taken to remove any loosened proteins or ground materials.

Tanning

The hides are first submerged in a very weak tanning liquor along with constant agitation to allow full permeation of the tanning liquor throughout the hide.

The wet hides are then laid in pits and layered with pulverized vegetable tanning materials and then with more hides, and then more tanning materials until the pit is filled. The pile is then topped off with either clean water or water mixed with extracted tanning materials and allowed to steep for approximately one year.

After necessary tanning time has passed, the hides are pulled from the pit and thoroughly rinsed and smoothed.

The hides are lastly dried very slowly in the dark.

Finishing

Scour

Smooth

Pare

Stretch

The fat liquoring step uses fats to impart suppleness to thinner leathers. Because oils and fats are hydrophobic and do not disperse in water, the natural oils (linseed, fish and castor are most common) are first sulphonated to form derivatives that are ionizable. These ionized derivatives (with -OSO2OH functional groups) can then react with water and form a dispersion. The level of sulphonation of the oils is directly related to the depth of penetration into the dermal fiber network, so more highly sulphonated oils penetrate more deeply into the leather.

Currying is a process similar to fat liquoring, but practiced only on thicker leathers (not on bookbinding leathers). The processes uses cod oil, paraffin oil and meat tallows, and is applied by hand. When leather is curried under tension, a high tensile strength, low stretch leather is produced.

Boarding involves rolling the hide repeatedly with hand rollers to soften and smooth the finish.

Staking (or perching) is another softening step which involved the rubbing of the flesh side of the hide on a large convex blunt blade.

Dying is done only as a near-final step. Historically, only natural dyestuffs were used.

18th-20th Century Industrial Changes to Vegetable Tanning

As the demand for leather increased, so too did the need for speed in the tanning process. Although the basic process of vegetable tanning changed little, many mechanized functions and more aggressive chemicals were introduced to speed the tanning process. Below are the many of the steps that were changed by mechanization or the introduction of chemical treatments:

Pretanning

Putrefication was carried out almost exclusively with alkaline baths.

Splitting was practiced to extend the square footage of tanned leather from each hide. Each hide can be split into a “grain split” and a “flesh split” hide, or paper-thin skiver. Splitting was done with band knives.

Pretanning

Fleshing also uses cylinder rollers and knife cylinders that are sharper than those used in hair removal.

Bating is accomplished by immersion in baths of proteolytic enzymes.

Drenching now uses acidic baths of lactic, acetic, or formic acid to lower the pH and break down protein structures.

Tanning

No major changes

Finishing

Currying is now done mechanically over a drum.

Physical Alterations Through Tanning

The sought after characteristics of leather produced through the tanning process are brought about by many physical and chemical changes to the untanned hide. Ideally, all leathers have an increased hydrothermal stability (an increased Ts), an improved softness and drape through the opening of the fiber structure and removal of the ground substance, are rendered imputrescible, and have fibers that do not stick together when wetted and dried. These changes are brought about through the combination of the many physical processes of tanning outlined above, but although through a series of chemical alterations to the collagen structure.

Increased Hydrothermal Stability

Hydrothermal stability is measured through the shrinkage temperature (Ts) of a hide. This is the temperature at which the energy input (heat) exceeds the energy bound in existing hydrogen bonding of the collagen structure resulting in the decomposition of the helical structure. By adding energy into the system that exceeds the energy tied up in the system through hydrogen bonding, the excess energy breaks hydrogen bonds that hold the helical structure in its partially extended form. When these bonds are broken, the helices coil more tightly and physically shrink. The resulting helical structure after the shrinkage temperature is reached will be a bit looser in shape and the collagen structure is chemically bonded through only covalent bonds and salt links, resulting in a less robust structure more susceptible to degradation. The shrinkage temperature for untanned hides is generally around 65°C (149°F).

The Ts can be increased through the process of tanning, and is often used as one of the primary measures of the efficacy of a particular tan. Tanning does not change the thermochemical processes of breaking hydrogen bonds, nor significantly alter the number of hydrogen bounds. Instead, the tanning process forms covalent cross-links between molecules within the triple helical structure, resulting in a stronger suspension of the partially extended helical structure. Therefore, the energy necessary to shorten the structure must now overcome not only the pre-existing hydrogen bonds, but also the covalent cross links formed during the tanning process.

An additional pathway for tanning to increase the Ts is through the alteration of the water matrix of the supramolecular structure. The water matrix for raw collagen is similar to a solvent shell where water molecules are bound to the outside of the structure, particularly at the hydroxyproline residue sites. This forms a sheath of water around the supramolecular structure, bound by hydrogen bonds. During the tanning process, much of this water matrix is replaced by tanning agents, thus changing the chemical composition of the matrix surrounding the structure and increasing the number of covalent bounds.

Degradation, either natural or chemically enhanced, can lower the Ts by reducing the number of existing hydrogen bonds. Natural degradation functions by intermittently breaking the molecular backbone via oxidation and reduction (hydrolysis) reactions, thus shortening the polypetide chain. Additionally, oxidation/reduction reactions also chemically alters the composition of the amino acid side chains, which can affect the number of hydrogen bonds. Liming, a process in the pretanning stages of leather manufacture to accelerate putrescence thus loosening hair follicles and unwanted interfibrillary proteins, also lowers the Ts. This is accomplished through the reaction of a strong alkali (often sodium sulfide) chemically reacting with the existing amino acid functional groups, thus changing their structure and reducing the number of hydrogen bonds.

Imputrescence

Imputrescence is achieved by removing many of the non-collagen substances found in untanned hides, but most importantly through...

fibers don’t stick when wetted and dried

Chamois…

Increased Softness and Drape

Liming step allows for the swelling of the pelt by the imbibition of water

The mucopolysaccharides in the ground substance are polyelectrolytes and bond water very firmly, so few other types of ions can reach the fibers of the dermal network while the ground is present.

With liming, the hydroxyl ions in the skin break down the bonds between the mucopolysaccharides and water, thus dispersing the ground substance and exposing the collagen fibers in the dermal network to hydroxl ions (and swelling).

Bating and/or deliming lower the pH and reduce the # of hydroxyl ions bound to the dermal network, reducing swelling and softening the pelt.

Use of Leather in Bookbinding

Leather has been used to cover all or parts of a book for as long as books have been bound. The most common use of leather for bookbinding is as a spine covering, but full leather bindings are also very popular, as are leather tips or corners. Leather can also be found in doublures, hinges, sewing supports, endbands, and various other pieces of the book anatomy due to its flexibility, durability, ease of decoration, and availability.

Historically, most leathers used for any purpose were vegetable tanned, with small portions being tawed, mineral, smoke or brain tanned. Books were bound in the leather available to them, but most historical exemplars were bound in alum tawed pigskin or vegetable tanned calf or goat. In the current day, almost 90% of all modern leather is chrome tanned, but book binders persist in a preference for vegetable tanned leathers for their workability, flexibility and strength.

The proceeding definitions of paring and leather decoration used in bookbinding are taken from Matt T. Roberts and Don Etherington's Bookbinding and the Conservation of Books, published 1982, and available digitally at [1]http://cool.conservation-us.org/don/don.html]

Paring

The process of thinning leather by cutting away the flesh side, or shaving the edges, i.e., beveling the edges that are to be turned in. A paring machine is generally used for the thinning process (or a SPOKESHAVE if no paring machine is available), while a paring knife is used for shaving or beveling.

Very little if anything is known of the method or methods used by binders to reduce thickness in the early days of covering books with leather, but it is entirely possible that from about the latter part of the 16th century they purchased leather from the manufacturer in the required thickness and then simply pared the edges.

During the 19th century there were no paring machines in use in binderies, nor were there any spokeshaves. There is no evidence of any paring of leather other than edges during the first half of the 19th century; consequently it must be assumed that the leather was purchased already pared, or was purchased and then sent out to be pared as required.

Decoration

Tooling

Finishing has assumed a very important role in the craft of bookbinding since the earliest times. Almost all early finishing, at least in Europe, was in blind until the latter half of the 15th century, when gold tooling was introduced into Italy. In modern finishing. all but the simplest designs are measured out and drawn or tooled on thin paper. This is then positioned on the cover and heated tools are pressed through. The paper is then removed, and the blind impressions are again blinded-in. This sharpens and deepens the impressions, and, if gold is to be used, provides a smooth flat surface for the metal. In addition to making it possible to execute extremely difficult patterns without making errors on the leather itself, the use of a paper pattern eliminates the necessity of making basic guide lines in blind upon which the design is then built, and which almost invariably show beyond the tooling. It is uncertain when paper patterns were introduced, but they probably were not used much before 1830. Not all leathers can be tooled successfully. Aside from the great difficulty encountered in tooling chrome-tanned leathers, only those vegetable-tanned (or tawed) leathers with surfaces firm enough to hold a line, such as goatskin, calfskin, pigskin, etc., are suitable. With the exception of sheepskin, leathers that are loose and stretchy do not retain impressions very well.

Gold ToolingThe art or process of lettering and/ or decorating the spine and covers of a book with GOLD LEAF (or, at times, other metals, e.g., platinum) impressed into the covering material, usually leather, by means of a heated letter, lettering pallet, or finishing tool. In the traditional method of gold tooling, the lettering or design is first blinded in, generally first through paper, and then again directly on the leather. The second working of the tool polishes the base of the impression and assists in creating a particular brilliance in the tooling. An adhesive (glair) is applied to the leather (either all over or directly into the blind impressions); strips of gold leaf are laid over the impressions and held in place temporarily with a thin film of vaseline or grease; and the gold is then pressed permanently into place with the heated tool When done properly, the affinity of the gold for leather is such that it will practically never come off; nor will it tarnish. Gold tooling must be ranked as one of the most important innovations in the history of bookbinding. Its origin are somewhat obscure, but it was probably introduced into Europe by way of Italy, and spread throughout the rest of Europe and England, eventually ar riving in America. There is some evidence that the technique may have been practiced in Morocco in the 13th century, but this is not conclusive. It has also been proposed that gold tooling was introduced into Italy by way of Persia (now Iran), where bookbinding and gilding flourished in the early decades of the 15th century. Very early gold tooling is difficult to evaluate because it is uncertain whether the gold was actually impressed into the leather with a (hot) tool, or was painted into blind impressions. The evidence offered by some bindings, i.e., the absence of impressions deep enough to indicate tooling, as well as what appear to be brush marks in the gold, would seem to indicate painting. Because of the elapsed time, however, which has led to the inevitable deterioration of the materials, it is difficult to differentiate between the two techniques. In any event, books were actually being tooled in gold in Venice no later than 1470, and possibly several years earlier. Gold-tooled leather bindings were not common in England before about 1530, and not in the United States until about 1669. The universal adoption of gold tooling was by no means immediate, and, in fact, blind tooling was still the predominant form of decoration until about 1580, or even 1600.

Blind ToolingA method of decorating a book in which impressions are made in the covering material, usually leather or tawed skin, by means of heated tools, pallets, rolls, fillets, or combinations of one or more of these. As the name implies, blind tooling does not entail the use of leaf metal, foil, or any other coloring material, with the possible exception of carbon, which is sometimes used to darken the impressions. The effect of blind tooling rests largely on the depth and uniformity of the impressions (which makes it unsuitable for use with hard covering materials) and the ability of the heated tool to produce a darkened color (see above)—factors which make leather, especially in the lighter shades, an ideal medium for this method of decoration. The critical aspects of the technique are the temperature of the tool and the degree of dampness of the leather. In general, the damper the leather the cooler the tool should be, and vice versa. In tooling leather blind, the surface is given a quick initial strike to "set" the leather in the impression. The tool is then impressed again and rocked slightly, which polishes and darkens the impression. When blind lines run across the spine of the book, polishing is accomplished by sliding a pallet along the lines; on the covers, where a fillet is used for long lines, it is fixed so that instead of rolling, it slides along the impression. Blind tooling has been used as a means of decorating books since the early days of bookbinding, and can be traced back to Coptic bindings of the 7th or 8th centuries, and even earlier. There is reason to believe that the technique was brought to Europe from the Mediterranean area about the same time as other Coptic techniques being used, possibly by imported craftsmen; however, little is known of blind-tooled bindings until the 12th century and early part of the 13th. In one form or another, the technique has been used continuously up to the present day, but during the 16th to 18th centuries, its use was more or less limited to inferior calf- and sheepskin bindings. Near the end of the 18th and during the early years of the 19th centuries blind tooling was often used on fine bindings in conjunction with gold. Also called "antique tooling."

Tree Calf

A form of cover decoration consisting of a smooth, light-colored calfskin treated with chemicals in such a manner as to represent a tree trunk with branches. In the usual manner, a dual design appears on upper and lower covers. The leather is first paste-washed, and the book is then hung between two rods which keep the covers flat. The book is tilted so that it inclines upward towards its head. In order to bend the boards outwards, i.e., warp slightly to a concave shape, so that the solutions will run properly, the insides of the hoards are not filled in until the decoration is done. A small amount of water is applied to the center of both covers to form the trunk, then more water is thrown on the covers so that it runs down to the trunk and to a central point at the lower edge of the hoards. Copperas (a green hydrated ferrous sulfate) is then sprinkled in fine drops on the covers, followed by potassium carbonate (salts of tartar). which causes the chemical reaction that etches the leather to form a permanent pattern in shades of gray, ranging from faint to very dark. Calfskin was used for this decoration in preference to sheepskin (although 19th century examples of sheepskin tree do exist) because in addition to being a much superior leather, it takes a better polish, which suits this style of decoration admirably. The spine of the book is protected during the marbling so that it will not be touched by the water or chemicals. The entire process calls for considerable experience and dexterity of execution, because if the result is to be effective the copperas and potassium carbonate must be applied in the correct amounts, as well as in the proper manner, while the initial water is still running down the covers; otherwise the effect will be little more than sprinkling. Late in the 19th century attempts were made to produce the tree calf effect with the use of an engraved block, which was used to print a design on the covers in black, but the results were ineffective because the block did not provide the shading which the genuine method achieved. The popularity of tree calf began to decline before the First World War, and by the late 1920s this once very popular form of decoration had virtually passed from existence. The first known tree calf decoration dates from about 1775.

Polishing

The process of smoothing and adding gloss to the covers of a book by mechanical means. The process involves working the leather at first with a slightly warm tool, followed by repeated workings with polishers heated to higher temperatures. Small circular motions are used to prevent wide areas of darkened streaks from showing. The technique of polishing leather covers dates back at least to the second half of the 14th century.

Leather Degradation

The complex structure of leather leads to challenges in discovering the exact decay pattern of a particular skin, which depends on several factors included in the leather processing as well as environmental conditions. Deterioration mechanisms and pathways are becoming more understood through the use of natural and artificial aging studies, primarily on vegetable tanned leather. Though there are great strides being made into the investigation of leather deterioration, there is much that remains unknown.

Breakdown of the collagen structure

Chemical Degradation

The two main mechanisms by which collagen degrades in leather are acid hydrolysis and oxidation, which affect not only the protein backbone of the collagen molecule, but also the tannins and lubricants used in the leather making process.

Acid Hydrolysis

The cleaving of bonds by acidic agents is termed acid hydrolysis. In collagen this breaking of bonds occurs between the amino acids along the protein backbone. Leathers contain a significant amount of acidic moieties, though sulfuric acid is thought to be the most prevalent player in acid hydrolysis. Sulfuric acid is thought to come primarily from atmospheric pollution, though there may be excess sulfur compounds, including sulfuric acid, from leather processing. Sulfur dioxide is prevalent in polluted air, and is absorbed into the leather. The sulfur dioxide is readily oxidized to sulfur trioxide, which combines with moisture to form sulfuric acid. Even though sulfuric acid is thought to be the primary acidic influence, other acids exist in leathers and contribute to hydrolytic breakdown. Environmental factors, such as high heat and high relative humidity will increase the rate of reaction for acid hydrolysis, see Figure 8.

[insert Figure 8: Hydrolytic breakdown of Collagen]

Acid hydrolysis in the protein chain results in the production of amine groups.

Oxidation

Oxidation and reduction involve a transfer of electrons and are frequently called redox reactions. Oxidation is the loss of electrons, even if oxygen is not present, and can be initiated by several factors, including, light, heat, free radicals, and oxygen. Oxidation by free radicals frequently results in a chain reaction. Results of redox reactions include both the breaking of bonds and polymerization. Metal ions and increases of relative humidity will serves as catalysts for oxidative deterioration.

Picture of Oxidative breakdown of Collagen**

Oxidative degredation of the protein chain results in amide groups, which eventually break down into ammonia. This ammonia can react with sulfuric acid and form ammonium sulphate thus sequestering the sulfuric acid and impeding deterioration by acid hydrolysis.

VI.A.4. Amino Acid degradation
Amino acids within the protein can deteriorate through three main pathways, deamination (loss of an amine group), decarboxylation (loss of a carboxylic acid group), and transamination (mutation of one amino acid to another). Researchers can use what they know of amino acid profiles of new and aged leathers to determine degradation pathways.

Analysis of Leather Degradation

Researchers characterize degree of degradation in a variety of ways, and some of the more common are mentioned here.

Degradation of larger polymers can be analyzed by looking at extractable monomers. The deterioration of tannins can be categorized in this way, and as mentioned above, amino acid profiles are used to look at very specific degradation of free and chain terminal amino acids.

Hydrolysis by sulfuric acid leads to the formation of sulfates. Analysis of sulfate contents has been used as an indicator of deterioration by acid hydrolysis (which does not take into account hydrolysis by other acids).

Shrinkage temperature is used as a measure of degradation of leather. It is the temperature at which leather shrinks under specified conditions. Shrinkage temperature can roughly be correlated to collagen denaturation, the breakdown of the higher structural levels of collagen. As the structurally supportive hydrogen bonds and tannin crosslinks break down, the collagen structure is disrupted and the shrinkage temperature decreases. There is a direct relationship between shrinkage temperature and degree of degradation.

Degradation properties of different tans

Vegetable Tannins

The vast majority of bookbinding leather is vegetable tanned. The two main types of vegetable tannin are condensed and hydrolysable. Most historic leathers will be tanned with some combination of tannins. Both condensed and hydrolysable tannins are weakly acidic phenolic compounds, which have anti-oxidant ability depending on their structure. The anti-oxidant capability of tannins is reduced upon exposure to strong oxidizing agents.

As tannins themselves degrade, the acidity of their environment is increased, which can increase acid hydrolysis reactions of other components of the leather. The crosslinks between tannin and collagen are broken, and the leather detans resulting in a decrease in shrinkage temperature decreases.

Hydrolysable (pyrogallols)

Hydrolysable tannins are those that when hydrolyzed break down to sugars and phenolic compounds, primarily galllic and ellagic acid. They are known historically to be more resistant to deterioration. Hydrolysable tannins have more naturally occurring salts and non-tans, which are thought to have a shielding effect against acid hydrolysis. Hydrolysable tannins also have a high degree of anti-oxidant ability, which protects them from oxidation.

Condensed (catechol)

Condensed tannins primarily degrade into insoluble compounds that are colored from yellow-brown to red. These tans have a reputation for higher degrees of deterioration than hydrolysable tannins. Condensed tans have a lower antioxidant activity and are more susceptible to oxidative degradation. Sulfur dioxide is readily absorbed by leathers tanned with condensed tannins, leaving them more vulnerable to acid hydrolysis.

Chromium

The bond between chromium tanning salts and collagen is much stronger than in vegetable tanned leathers. This results in a higher shrinkage temperature and leather that is more resistant to deterioration. Chrome-tanned leathers do not absorb large amounts of sulfur dioxide and do not show signs of appreciable degradation in accelerated aging trials. The working properties of chrome-tanned skins are not ideal for bookbinding.

Alum-tawed

Alum-tawed skins, while not truly “leather”, are used fairly heavily in bookbinding. These skins have a high concentration of alum and salts that are thought to be protective against deterioration. Since the alum salts are not bonded to the collagen, and can be washed out with water, effectively “detanning” the skin. Alum-tawed skins do not take up significant concentrations of sulfur dioxide and have proven to be very resistant to degradation.

Combination Tans

Working on combination tannages to combat deterioration, thus far they look like….

Macroscopic Effects of Degredation

Red Rot

catechol and its degredation products

Spue/Bloom

waxy vs. salt Include in here the thought that salts protect from deterioration

Blackening of Leather

excess tans as well as non-tans migrate to the surface of the leather upon introduction of water or other polar solvents

Report of a discussion group session held at the 2001 AIC Annual meeting in Dallas. Describes joint tacketing, Japanese paper hinge repair, board slotting, and other techniques. Rebacking as a technique was excluded.

Grattan, David W. “Oxidative Degradation of Organic Materials and Its Importance in Deterioration of Artifacts” Journal of the International Institute for Conservation — Canadian Group 4, no. 1 (autumn, 1978): 17-26.

Larsen, Rene (ed.) STEP Leather Project: Evaluation of the Correlation between Natural and Artificial Ageing of Vegetable Tanned Leather and Determination of Parameters for Standardization of an Artificial Ageing Method Protection and Conservation of European Cultural Heritage Research Report No. 1, The Royal Danish Academy of Fine Arts, Copenhagen, Denmark, 1994?

Smith, Richard Daniel. The Preservation of Leather Bookbindings from Sulfuric Acid Deterioration. A Research Paper Presented to Dr. Bohdan S. Wynar in partial fulfillment of a Master of Arts in Librarianship. University of Denver, 1964.

Report of a survey on leather conservation practice, focusing on special collections library treatments. Discusses frequency of use of types of repair materials, techniques, adhesives, toning, and surface treatments. The author concludes that the conservators surveyed employed a wide variety of techniques, increasingly choose minor mends over rebacks and rebindings, and have adopted newer materials like Japanese tissue, Klucel G, and waxes.